Hello @Yamazaki
Welcome to Microsoft Q&A community forum!
We have not added support for B-series VMs as an option in Azure migrate assessments yet. This is lower in priority in our current backlog. I would encourage you to file a feature item at our User-Voice forum to let the team know of your need for the feature. They regularly review feedback there for prioritizing work and features. Also, up-vote the feature - The more individuals that vote for the request, the higher the likely hood it will be implemented.
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If the response helped, do "Accept Answer" and up-vote it